1. Field of the Invention
The present invention relates to a control circuit for a semiconductor device with overheat protecting function.
2. Description of the Related Art
FIG. 3 is a circuit diagram showing a prior art control circuit for a semiconductor device with overheat protecting function.
The control circuit consists of a microcomputer 1, an amplifying circuit 2, a semiconductor device 3 with overheat protecting function, and a the load 4.
The microcomputer 1 generates therein a control signal, for example a pulse-width modulation control signal (hereinafter, PWM control signal), to carry out control of the semiconductor device 3 with overheat protecting function, or receives it from a non-shown external signal source, and then output it from an output port P1.
The amplifying circuit 2 a push-pull type amplifying circuit consisting of transistors 21, 22, 23, 24 and resistances 25, 26, 27, 28, 29, 30, 31, 32.
The semiconductor device 3 with overheat protecting function is of a MOSFET (i.e. metal oxide semiconductor field-effect transistor) with overheat protecting function, and consists of a MOSFET 33, a gate resistance 34 connected between a gate of the MOSFET 33 and a gate terminal G, a temperature detecting circuit 35 connected between a source terminal S and a source of the MOSFET 33, a latch circuit 36 to latch a temperature-detection output of the temperature detecting circuit 35, and a gate breaking circuit 37 to be controlled with an output of the latch circuit 36 connected between the gate and source of the MOSFET 33, which all are mounted on a chip. A drain of the MOSFET 33 is connected to a drain terminal D connected to a xe2x80x9c+Bxe2x80x9d power source, and the source is connected to the source terminal S.
An overheat protecting function of the semiconductor device 3 is carried out by the gate resistance 34, the temperature detecting circuit 35, the latch circuit 36 and the gate breaking circuit 37.
The load 4 is, for example, a lamp in use for a blinker of a vehicle.
In the above-described structure, the PWM control signal is output from the output port P1 of the microcomputer 1, is amplified by the amplifying circuit 2, and is supplied to the gate terminal G of the semiconductor device 3 with overheat protecting function through the resistance 32.
In a normal operation state, the MOSFET 33 gets ON/OFF by the PWM signal supplied to the gate terminal G, and an amplified PWM control signal arises on the source and is supplied to the load 4 through the source terminal S. The load 4 is driven by the supplied PWM control signal.
On the other hand, when an abnormality such as the short of the load 4 arises a large current flows between the source and the drain of the MOSFET 33 due to the PWM control signal coming after that occurrence in the first place, whereby temperature of the chip rises. The rise in temperature of the chip is detected by the temperature detecting circuit 35, and the detection output is supplied to the latch circuit 36. Subsequently, the output of the latch circuit 36 is supplied to the gate breaking circuit 37. The gate breaking circuit 37 is controlled by the output of the latch circuit 36 so as to protectively intercept a gate input of the MOSFET 33.
Accordingly, the large current to the MOSFET 33 stops, and temperature of the chip drops.
After the above protective operation, the control of the latch circuit 36 and the gate breaking circuit 37 are released with the rise voltage of the PWM control signal coming next, whereby the large current flows between the source and the drain of the MOSFET 33 again and temperature of the chip rises again. The rise in temperature of the chip is detected by the temperature detecting circuit 35, and the latch circuit 36 and the gate breaking circuit 37 protectively operate with the detection output. The protective operation is carried out like above every time the PWM control signal rises.
As a result, as is shown in FIG. 4, when the start time of the gate voltage of the MOSFET 33 by the PWM control signal supplied from the microcomputer 1 through the amplifying circuit 2 is designated as Tg(on) and the fall time of the gate voltage is designated as Tg(off), the source voltage waveform is the same as the gate voltage waveform in the normal operation.
With respect to the above prior art, however, when the short in the load 4 arises, the pulse width of the source voltage waveform gradually becomes short every time the protective operation occures.
And, since the overheat protecting function has limitation in frequency structurally, the MOSFET could be broken in several seconds to several minutes in case the short in the load 4 has occurred in a state that the PWM control signal is applied.
In view of the foregoing, an object of the present invention is to provide a control circuit for a semiconductor device with overheat protecting function wherein the risk of breakage of the semiconductor device can be reduced.
In order to achieve the above object, as a first aspect of the present invention, a control circuit for a semiconductor device with overheat protecting function comprises: a semiconductor element; an overheat protecting means; a chip to mount the semiconductor element and the overheat protecting means; a control means to supply a pulse-width modulation control signal having a fixed pulse width to the semiconductor element; and an outputting state detecting means to detect abnormality of output of the semiconductor device with overheat protecting function during an overheat protective operation of the overheat protecting means, wherein the control means monitors detection output from the outputting state detecting means at a fixed monitoring timing and stops supplying the pulse-width modulation control signal to the semiconductor element when the detection output is generated successively predetermined times or successively during predetermined time.
As a second aspect of the present invention, in the structure with the above first aspect, the semiconductor element is of a MOSFET, and the overheat protecting means includes a temperature detecting circuit, a latch circuit, and a gate breaking circuit.
As a third aspect of the present invention, in the structure with either of the above first or second aspect, the control means monitors the detection output from the outputting state detecting means at each monitoring timing of the pulse-width modulation control signal""s start time plus a fixed time period being shorter than the fixed pulse width.
According to the above-described structures of the present invention, the following advantages are provided.
(1) Since the operation state of the semiconductor device with overheat protecting function is detected, the risk of the breakage of the semiconductor device can be greatly reduced. And, in case that the semiconductor device with overheat protecting function is used for driving a lamp of a vehicle, occurrence of the fire in the vehicle can be prevented.
(2) The risk of the breakage of the MOSFET can be greatly reduced.
(3) The monitoring timing can be suitably set.
The above and other objects and features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.